1. Field of the Invention
This invention relates to an optical distance-measuring apparatus and more particularly to a method and apparatus for eliminating leakage signals which may reduce the measurement accuracy.
2. Description of the Prior Art
An optical distance-measuring apparatus emits a distance-measuring light from its body and receives the distance-measuring light reflected from a corner cube, for example, which is installed at a point whose distance is to be measured and the distance to that point is measured accurately by the phase difference between the emitted light and the received light. For this purpose a light-emitting element consisting of a light-emitting diode or a laser diode is provided in the body of the optical distance-measuring apparatus, and this light-emitting element generates optical waves modulated through an electric circuit of a transmitting system and projects them onto a reflector installed at the measurement point through a mirror and an objective lens. The optical waves reflected by the reflector are received by a light-receiving element consisting of a photodiode through the objective lens and a second mirror, and its signal is subjected to a given processing in an electric circuit of a receiving system.
The optical path from the light-emitting element to the light-receiving element through the mirror, objective lens, reflector, objective lens and second mirror constitutes an external distance-measuring optical path contributing directly to the distance measurement. On the other hand, an internal reference optical path aimed directly at the light-receiving element from the light-emitting element via another mirror is formed within the body. Since response delays and temperature drift in the various electric circuits constituting the optical distance-measuring apparatus are capable of influencing the phase difference measurement which is essential for data on the distance measurement, the internal reference optical path is designed to eliminate these influences of the electric circuits. The phase difference between the modulated waves from the transmitting system and the modulated waves received by the receiving system is subjected to an arithmetic operation in a processing circuit and is displayed as measured distance data. The external distance-measuring optical path and the internal distance-measuring optical path are interrupted by a chopper. The optical path interruption chopper used hitherto for this purpose consists of an opening forming an internal reference domain for selecting the internal reference optical path and a peripheral notch forming an external distance-measuring domain for selecting the external distance-measuring optical path, and is so constituted that it selects either of the optical paths by rotating round its center to present the external distance-measuring domain and the internal reference domain from the light-emitting element alternately. Both the external distance-measuring optical path and internal reference optical path are necessary for the distance measurement, they are called "normal optical path" hereinafter.
In the optical distance-measuring apparatus, electric leakage signals caused by electric fields, magnetic fields, electromagnetic waves, etc., may also exert a large influence on the distance-measuring precision in the electric circuit comprising the transmitting system, receiving system, precessing circuit, etc., which constitute the apparatus.
There are also leakage optical paths in the optical distance-measuring apparatus reflecting from the optical path interruption chopper, bearing mountings of the optical parts, internal walls of the apparatus, etc., and reaching the light-receiving element, and leakage optical paths reflecting from the optical parts themselves and reaching the light receiving element other than by the above normal optical path.
In the receiving system, a leakage signal S.sub.l in which the above optical leakage signals and electrical leakage signals are combined is given by: EQU S.sub.l =l sin (.omega.t+.psi..sub.l) (1)
where
l: amplitude PA1 .psi..sub.l : phase PA1 .omega.:.omega.=2.pi.f (f being the modulation frequency) PA1 t: time. PA1 d: amplitude PA1 .psi..sub.d : phase PA1 .omega., t: same as in equation (1).
On the other hand, a normal signal S.sub.d coming from the normal signal system uninfluenced by the leakage signal by way of the normal optical path which is the external distance-measuring optical path or the internal reference optical path is given by: EQU S.sub.d =d sin (.omega.t+.psi..sub.d) (2)
where
Therefore, the actual signal obtained by the receiving system during the distance measurement is a composite signal S.sub.h of the leakage signal S.sub.l and the normal signal S.sub.d combined. If the composite signal S.sub.h has the relationship .psi..sub.l =.psi..sub.d +.DELTA..psi. between the phase .psi..sub.l of the leakage signal and the phase of the normal signal, then ##EQU1## The above h indicates the amplitude of the composite signal S.sub.h, and .psi..sub.h indicates the phase increment due to the leakage signal in the composite signal S.sub.h. In an optical distance-measuring apparatus obtaining a distance through the result of a phase measurement, the presence of .psi..sub.h leads directly to an error.
Usually the strength of the leakage signal S.sub.l is very small when compared with the normal signal S.sub.d so that the condition d&gt;&gt;l can be set between the amplitude d of the normal signal S.sub.d and the amplitude l of the leakage signal S.sub.l. Therefore, h, .psi..sub.h in the above equation (3) are given by: ##EQU2##
.psi..sub.d will vary from 0 to 2.pi.(rad) according to the distance being measured. Therefore, as will be apparent from .psi..sub.h of equation (4), the measurement error varies as a sine function of the distance being measured. The error is a maximum at l/.sub.d (rad), therefore when it is converted into a distance, the error in the measured distance .theta..sub.l is ##EQU3## where .lambda.: modulated wavelength.
When, for example, a modulated wave of 15 MHz (.lambda.=20 m) is used and the amplitude ratio l/.sub.d of the leakage signal to the normal signal is assumed to be 1/200, .theta..sub.l .apprxeq.8 mm and the error expressed as a sine function has a maximum of +8 mm and a minimum of -8 mm.
In an optical distance-measuring apparatus which is required to measure a distance of several kilometers accurately to millimeter units, it may be understood that the presence of a leakage signal will be serious enough to bring about a large error in the result of the distance measurement.
In a conventional type of optical distance-measuring apparatus, the following measures have been taken to prevent such a leakage signal from occurring. First, to prevent the leakage optical paths which generate optical leakage signals, a diffusion surface treatment is applied to the optical path interruption chopper, the bearing mountings of the optical parts, or the internal walls of the apparatus, or shading plate are provided at all the necessary spots, though a satisfactory effect has not been obtained thereby.
The space necessary for mounting the shading plate etc., must be provided, which increases the size of the optical distance-measuring apparatus.
To prevent electrical leakage signals, the transmitting side and the receiving side are isolated by a shield, and to prevent leakage from the power line a filter or the like is used. In particular, the electromagnetic waves generated by the power amplifier driving the light-emitting element is extraordinarily influential when compared with the other circuits, and the preamplifier amplifying minute signals from the light-receiving element is most sensitive to a leakage signal, so that the power amplifier and the preamplifier are each sealed independently from the shielding structure, single or dual as occasion demands, though the effects are yet unsatisfactory. Furthermore, these shading plate and shielding structure have the drawback in that they necessitate an increase in the number of parts and construction steps, and eventually increase the price of the apparatus. Since the optical distance-measuring apparatus is intended for outdoor surveys, the apparatus itself must be portable, small and lightweight, however the above shielding structure and multitude of dousers that must be incorporated are contrary to the requirements of small size and light weight for the apparatus.